(1) Technical Field
This invention generally relates to electronic circuits, and more specifically to circuitry and methods for sensing the state of fusible link components used in such electronic circuits.
(2) Background
Fusible link technology in electronic circuits, particularly integrated circuits (ICs), has been in use for some time. Fusible links (also called just “fuses”) are used for numerous purposes, such as chip IDs, serial numbers, and trimming. In particular, fuses are often used in electronic circuits to permanently or semi-permanently set values or states for other components within the circuit. For example, in some applications, fuses may be used to set a value for a tunable circuit element such as a digitally tunable capacitor of the type taught in U.S. patent application Ser. No. 12/735,954, Publication No. 20110002080A1, entitled “Method and Apparatus for Use in Digitally Tuning a Capacitor in an Integrated Circuit Device”, filed on Mar. 2, 2009 and assigned to the assignee of the present invention. Such tunable circuit elements often need to be tuned only once during manufacture in order to achieve a particular design specification despite process variations and other factors that may otherwise cause circuit performance to vary from circuit to circuit. Accordingly, once such a tunable circuit element is calibrated to a desired performance level or configuration, then that static value can be embodied in a permanent or semi-permanent form using fusible links.
In general, a fuse comprises a normally conductive (“unblown”) member for electrically interconnecting other circuit elements. However, the conductive member can be melted, disintegrated, or fractured (“blown” or “burned”) by means of a pulse of electrical current to create an open circuit (in theory). Another method of blowing or burning a fuse, called electro-migration, involves applying a current at a level that keeps the conductive member from exploding, but still keeps it molten so that the conductive material gets carried away, outside a narrow region. This method allows for a wider section of conductive material to be displaced, and is believed to provide for a more robust product lifetime.
In some IC applications, significant quantities of fuses are required, and fuse reliability is absolutely necessary. It is therefore often critical that the actual post-production state—blown or unblown—be determined for all fuses; it is not sufficient simply to assume that a fuse that was supposed to have been “blown” was in fact fully blown, or that a fuse that was not supposed to have been “blown” was in fact unblown. Accordingly, it is generally necessary to sense the state of fusible link components used in such electronic circuits in order to verify the fusible link state (i.e., blown or unblown).
Unblown fuse resistance may typically be less than 100 ohms, whereas blown fuse resistance may be anywhere from 700 ohms if blown incompletely (i.e., partially blown) to somewhere between 4000 ohms and infinity (i.e., an open circuit) if blown completely. To sense the value of a fuse, a reference resistor is typically utilized to compare against the fuse resistance: while applying a current, if the reference resistor is higher in resistance, the fuse was not blown, whereas if the fuse is higher in resistance than the reference resistor, the fuse was blown. Commonly, to do a typical resistor comparison of this sort, some reference current is imposed on both the reference resistor and the fuse, and the difference (“delta”) in voltage across the two devices is converted to digital logic levels (“0” or “1”) and stored (for example, in volatile memory).
FIG. 1 is a schematic diagram of one example of a prior art fuse sensing circuit 100. A fuse 102 is coupled between a corresponding reference current source 104a and circuit ground. In addition, a reference resistor Rref 106 is coupled between a corresponding reference current source 104b (which may be the same source 104a coupled to the fuse 102) and circuit ground. When essentially the same current is applied to both the fuse 102 and the reference resistor Rref 106, the voltage across each element relative to circuit ground can be measured by a conventional test instrument or device or other circuit arrangement (not shown). The difference ΔV between the two measurements will determine whether the reference resistor Rref 106 is higher in resistance than the fuse 102 (indicating that the fuse 102 was not blown), or if the fuse 102 is higher in resistance than the reference resistor Rref 106 (indicating that the fuse 102 was blown). In a typical application, if ΔV indicates an unblown state, a logic “0” is stored in an associated latch circuit, and if ΔV indicates a blown state, a logic “1” is stored in the associated latch circuit. At a later time, the latched value may be read back to determine the previously measured state of the fuse.
The amount of current used for measurement cannot be too high because otherwise the current may damage (i.e., partially “blow”) an unblown fuse. Thus, a limit is generally set on how much differential voltage can be created without damaging a fuse. In a typical example, the reference resistor value is set about half way between the expected blown and unblown fuse resistance values. For example, if the minimum standard for considering a fuse to be blown is about 2100 ohms and the unblown resistance value for the fuse is expected to be about 100 ohms, then 1100 ohms may be a good choice for the reference resistor (i.e., about 1000 ohms from the end-point fuse resistance values of 100 ohms and 2100 ohms). If the maximum current that the fuse can tolerate is around 150 μA, that leaves an error margin of about 150 mV of differential voltage to be sensed (1000 ohms×150 μA=150 mV). The minimum sensed voltage has to be large enough, and the sense circuit has to be accurate enough, that there are no mistakes in the reading. In the example given here, the voltage across a blown fuse should be about 15 mV (100 ohms×150 μA), whereas the voltage across an unblown fuse should be about 315 mV (2100 ohms×150 μA). Accordingly, it is relatively easy to distinguish those voltages from the voltage across an 1100 ohm reference resistor (1100 ohms×150 μA=165 mV).
Sometime a fuse is not blown properly due to defects in the fuse or fuse burning circuitry, or, in the case of electro-migration, because a sufficient amount of conductive material has not migrated. Further, a fuse may not be read correctly because sense circuit or other problems may occur in the test setup. If a blown fuse value is still above the reference resistor value but only by a small amount, there may be circumstances where the fuse resistance value would read properly in manufacturing screening, but fail (e.g., be read as not blown) in the field, particularly if different testing equipment is used. Reducing the value of the reference resistor is one option, but doing so decreases the noise margin for determining that a fuse is unblown.
Accordingly, there is a need for a reliable way of sensing the state of fusible links, some of which are unblown and some of which are supposed to have been blown. The present invention addresses this need.